1. Field of the Invention
The present invention relates to a semiconductor display device using a protective circuit which can prevent a dielectric breakdown of a light emitting element from occurring.
2. Description of the Related art
How to restrain degradation of the light emitting element or charging phenomena (charging) which leads to dielectric breakdown is an important subject in a manufacturing step of a semiconductor device. Specifically, with high integration, there is a tendency of not only miniaturization of a channel length but also decrease in film thickness of various insulating films typified by a gate insulating film. Consequently, the dielectric breakdown due to the charging becomes a more serious problem.
The cause and environment to occur the charging is extremely complicated and vary. Therefore, it is necessary not only to investigate the cause and the environment to occur the charge, but also to take measures in a structure of the semiconductor device so as to enhance resistance to the degradation or dielectric breakdown due to the charging. In order to prevent the degradation or dielectric breakdown due to the charge, it is effective to ensure a discharge path by the protective circuit using a diode (protective diode). By ensuring the discharge path, charge accumulated in an insulating film is prevented from being discharged in the vicinity of the semiconductor element. Thus, phenomena (ESD: Electro-Static Discharge) in which the semiconductor element is deteriorated and damaged due to energy of discharge can be prevented.
In addition, by providing a protective circuit, even if noise is inputted into a wiring along with a signal and a power supply voltage, a subsequent circuit can be prevented from malfunctioning due to the noise, and a semiconductor device can be prevented from being deteriorated or a damaged due to the noise.
In a semiconductor display device typified by a liquid crystal display device and a light emitting device, an active matrix type in which supply of signal to a display element can be maintained to some extent after inputting a video signal can respond flexibly to a panel with a big size and high precision, and thus the semiconductor display device with an active matrix type has become future mainstream. A structure of a pixel in an active matrix type semiconductor display device which is proposed concretely depends on each of manufacturer in a measure. Usually, a display element such as a light emitting element and a liquid crystal element, and a thin film transistor (hereinafter referred to as TFT) for controlling operation of the display element are provided for each of the pixels.
In the semiconductor display device, there are two kinds of structures that a display element is formed with a wiring connected directly to a first or a second terminal of a TFT over an insulating film (a first interlayer insulating film) which covers the TFT, and that a display element is formed over an insulating film (a second interlayer insulating film) which covers the wiring moreover. In the case of a light emitting device where light from the light emitting element is taken out from the side opposite to the TFT, the latter structure is more desirable for being able to increase contrast more than the former structure since ratio of a region which contributes to luminescence in the entire pixel portion can be increased.
The second interlayer insulating film covering the wiring moreover has a possibility of affecting characteristic of the display element due to its surface unevenness, and therefore, is formed by using a coating method which is easy to flatten. This insulating coating film has a problem of being easily charged when it is formed. However, a TFT is usually used for a protective diode, and the TFT is formed at the same layer as TFTs constituting other circuits. Accordingly, the charge which is charged in an insulating film in a layer formed over an insulating film which covers the TFT used for the protective diode is hard to discharge through a discharge path secured by the protective circuit. Consequently, there has been a problem that dielectric breakdown easily occurs due to the charge.